As a device to be mounted on an internal combustion engine, a variable valve device for changing a valve timing of an intake valve and an exhaust valve that are driven to open and close by a camshaft, according to an engine operating state, is known (refer to Patent Document 1, for example).
The structure of a conventional and general variable valve device, including the one described in the Patent Document 1, is shown in FIG. 14. FIG. 14(a) shows the internal structure of a variable valve device 100 from which a cover is removed, and FIG. 14(b) shows the cross-sectional structure taken along line B-B in FIG. 14(a). Incidentally, the rotational direction of a camshaft 200 is shown by arrow RC in FIG. 14(a).
The variable valve device 100 shown in FIG. 14 has two rotating bodies rotating about the same axis of rotation. One of the rotating bodies include a sprocket 101 drivingly connected to a crankshaft via a chain (illustrations omitted), a housing 102 fixed thereto, and the other rotating body includes a vane rotor 103 drivingly connected to the camshaft 200. A plurality of vanes 103A provided on the vane rotor 103 are respectively received in a plurality of housing chambers 105 formed inside the housing 102, and each housing chamber 105 is divided by the corresponding vane 103A into an advancement chamber 106 and a retardation chamber 107. A hydraulic pressure supplied to the advancement chamber 106 and the retardation chamber 107 displaces the vane 103A in the housing chamber 105 and rotates the housing 102 and the vane rotor 103 relative to each other, so that the relative rotational phase of the camshaft 200 relative to the crankshaft, that is, the valve timing is changed.
Further, the variable valve device 100 is provided with a lock mechanism 110 capable of fixing the valve timing at a timing that is between the most retarded timing and the most advanced timing and at which the engine can be started and, more particularly, at a timing capable of starting the engine in a cold start condition (hereinafter referred to as a specific timing). As shown in FIG. 14(b), the lock mechanism 110 includes a recess 112 formed in the sprocket 101, and a lock pin 111 received in a vane 103A to be able to approach and separate from the recess 112. In a space for receiving the lock pin 111 in the vane 103A, a spring 113 for urging the lock pin 111 and a release chamber 114 into which hydraulic oil is supplied with a predetermined hydraulic pressure are formed. The lock pin 111 is urged by the spring 113 in a direction to be fitted into the recess 112, whereas the lock pin 111 is urged by force based on the pressure of the hydraulic oil supplied to the release chamber 114 in a direction to be pulled out from the recess 112.
When conditions for locking the valve timing at the specific timing are satisfied, such as when an engine stop request is made, the hydraulic oil is drained from the release chamber 114. When the hydraulic pressure of the release chamber 114 is reduced correspondingly to be lower than a release oil pressure, the lock pin 111 fits into the recess 112 by the urging force of the spring 113, and the relative rotation between the vane rotor 103 and the housing 102 is mechanically locked. As a result of this, the valve timing is locked at the specific timing. Consequently, favorable engine startability can be secured at the next engine start-up time as cranking is started while the valve timing is at the specific timing.
Meanwhile, when conditions for releasing the valve timing from the specific timing are satisfied, such as when a change request of the valve timing is made, the hydraulic oil is supplied to the release chamber 114. When the hydraulic pressure of the release chamber 114 is increased correspondingly to be higher than the release oil pressure, the lock pin 111 is pulled out from the recess 112 by the urging force based on the hydraulic pressure, and the locking of the relative rotation between the vane rotor 103 and the housing 102 is released. As the hydraulic oil is selectively supplied to and drained from the advancement chamber 106 and the retardation chamber 107, the valve timing that has been locked at the specific timing is changed to a timing suited for the engine operating state.